1. Field of the Invention
The present invention relates to a small-sized imaging lens which improves the assembling property without deteriorating the performance thereof.
2. Description of the Related Art
A conventional small-sized imaging lens has been designed for use with a simple camera in the form of what is called a film with a lens. Japanese Unexamined Patent Publication Nos. 4-211215, 5-281465 and 6-258574, for example, disclose an imaging lens configured of two pieces of lenses.
Also, other applications of the imaging lens having two lenses as described in the above-mentioned publications include those described in Patent Publication No. 8-5910 used for a read optical system such as a facsimile and Patent Publication No. 5-341185 for the endoscope.
By the way, the conventional small-sized imaging lenses described above have the following problems.
Specifically, the imaging lenses described in Patent Publication Nos. 4-211215 and 5-281465, in which the image surface is corrected by the curving of the surface of a silver salt film providing a photosensitive material and the open F-number is as dark as about 9 to 11, is not suitable as an imaging lens for an image-pickup equipment using an imaging device (hereinafter called the CCD) in place of the silver salt film.
The imaging lens described in Patent Publication Nos. 6-258574 and 8-5910 is what is called of the telephoto type configured of a lens having a positive refracting power and a lens having a negative refracting power sequentially from the object side in order to reduce the size. Therefore, the angle of the emitted light ray is so large that a color shift is caused by the use of the CCD as a photosensitive member. Further, it is difficult to secure a space for filters for cutting the infrared light and a low-pass filter essential to the image-pickup equipment using the CCD.
Also, the imaging lens described in Patent Publication No. 5-341185 designed for the endoscope is configured of lenses having a negative refracting power and a positive refracting power arranged sequentially from the object side. An iris is arranged between these lenses and the CCD is used as a photosensitive member. Nevertheless, the distortion is caused by the asymmetric arrangement of refractive power with respect to the iris, and the eccentricity of the two lenses deteriorates the optical performance, thus requiring a high assembly accuracy.